Looking at the attacker's address in Omni Explorer: https://omniexplorer.info/lookupadd.aspx?address=16tg2RJuEPt...

I'm finding this a little confusing as it looks like BTC blockchain addresses can also be Tether addresses. Is this correct?

They are currently holding $30.9M USDT. If they transfer this to another USDT address, I assume it would show up here, like the 3 inbound tx from 31okFF1rUu8jjPEVuajycTRBp82Nteo4Mv that makes up their balance.

But what if they want to cash out to BTC? If they bought on Bitfinex with their USDT would another transaction appear? And could it then be tracked to the BTC blockchain explorer?

It was claimed that the stolen USDT could be mixed into the main supply of BTC, but I'm failing to understand how that would happen, as it seems traceable from the public addresses.